1. Field of the Invention
The present invention relates to a magnetic actuator having a plate, which is mounted rotatably about at least one axis of rotation.
2. Description of the Related Art
There are various approaches for magnetically driving micromirrors. The essential advantage of a magnetic drive in comparison with an electrostatic drive is that the torques technically achievable through the magnetic Lorentz force are higher than the torques achievable by electrostatic force.
Published European Patent document EP 778657 B1 proposes a gimbal-mounted mirror which is mounted rotatably about two axes of rotation, an internal axis and an external axis. The axes of rotation are perpendicular to one another. One coil for the internal axis and another coil for the external axis are situated on two gimbal-mounted vibrating bodies. The magnetic field is approximately homogeneous over the entire chip and forms a 45° angle to each of the two axes of rotation. Two configurations are proposed for generating the magnetic field. In a first variant, a micromirror is placed at a 45° angle between two hard magnets (permanent magnets). In a second variant, four magnets of opposing polarities are used. The effective magnetic field, which is available for the corresponding axes, is reduced by a factor of √2 due to the 45° angle. Both variants of the magnetic field generation require a very voluminous configuration.
Published international patent application document WO 2005/078509 A2 also proposes a gimbal-mounted mirror which is mounted rotatably about two axes. However, with this device only one coil is provided for the drive about two axes. A low-frequency quasistatic signal and also a high-frequency resonant signal are applied to this coil. The resonant signal excites the rotation of the internal axis via the so-called “rocking mode.”
Published international patent application document WO 2010/065340 A2 describes the relatively complex configuration of permanent magnets of differing polarities and flux conduction layers for generating a magnetic field in a 45° direction. The effective magnetic field perpendicular to the corresponding axes is likewise reduced by the factor of √2.
Published German patent application documents DE 102008042346 A1 and published European patent application document EP 1858141A2 propose configurations for generating a magnetic field component or a torque perpendicular to the quasistatic axis, so that when a high-frequency alternating field is applied, a resonant movement about axis 4b, which is perpendicular to the quasistatic axis, is induced.
In these specific embodiments of a micromirror driven by Lorentz force, unidirectional magnetic fields running in the plane of the chip surface are implemented. A deviation from the preferential direction constitutes an incomplete embodiment. The technical efforts are concentrated on minimizing such incomplete specific embodiments. To achieve a high magnetic field having the greatest possible unidirectionality, the magnetic flux conductor components used are relatively voluminous.
A radially symmetrical magnetic field for implementing a biaxial microscanner is proposed in the publication “Silicon scanning mirror of two DOF with compensation current routing”; Si-Hong Ahn and Yong-Kweon Kim; 3. Micromech. Microeng. 14 (2004) 1455-1461. The radially symmetrical field is implemented by a magnet, whose direction of magnetization is perpendicular to the plane of the chip. The configuration is thus relatively simple, which is an advantage with respect to the manufacturing costs. The fountain shape of the magnetic field lines includes a radial component running in the plane of the chip. Only this component is technically usable, while some of the other components are not usable and some would even lead to undesirable transverse forces. The usable magnetic field strength claimed in the publication is 0.1 T.
The magnetic field is increased by a configuration of two magnets in the publication “Electromagnetic Two-Dimensional Scanner Using Radial Magnetic Field”; Chang-Hyeon Ji, Member, IEEE, Moongoo Choi, Sang-Cheon Kim, Ki-Chang Song, Jong-Uk Bu, Member, IEEE, and Hyo-Jin Nam; Journal of Microelectromechanical Systems, vol. 16, no. 4, August 2007. The central cylindrical magnet, whose direction of magnetization is perpendicular to the chip surface, is surrounded by a ring-shaped magnet of the opposite polarity. Both magnets are secured on a disk-shaped iron plate, so that a portion of the magnetic flux is returned. The usable magnetic field was increased up to 0.5 T by this configuration, but this design is just as complex as that of scanners, which operate on the basis of a unidirectional magnetic field.
The packaging must be taken into account in ascertaining the manufacturing price of a micromirror. Mounting of magnets during assembly is assessed as being very cost-intensive. Mounting of magnets of differing polarities in particular requires even greater effort during assembly due to the mutual repulsion.
If a hard magnet having one direction of magnetization is used in the case of a magnetic yoke, for example, then the remagnetization may take place after assembly, which would greatly reduce the effort and thus the costs.